1. Field of the Invention
Embodiments of the invention are directed to logging tools. More particularly, embodiments of the invention are directed to pulsed-neutron logging tools, possibly used to determine formation lithology and/or macroscopic capture cross-section.
2. Background
Pulsed-neutron logging tools may be used in uncased boreholes, cased boreholes, and production tubing of a cased borehole. Pulsed-neutron logging tools operate by releasing high energy neutrons, on the order of 14 Million electron Volts (MeV), into the borehole and formation. The neutrons create gamma particles or gamma rays by one of several of reactions: 1) the neutrons inelastically collide with atomic nuclei and thereby create gamma rays (known as inelastic gamma rays); and 2) when a neutron loses energy (possible through the inelastic collisions or by other means) the neutron is captured by a nearby atom and in the process a gamma ray is emitted (known as a capture gamma ray). When enough gamma rays are observed to obtain a statistically significant distribution, inelastic gamma rays and capture gamma rays and their element of provenance can be distinguished by their energy.
When using a pulsed-neutron logging tool as a bulk density measurement device, inelastic gamma rays carry most of the information as to the formation bulk density. When using a pulsed-neutron tool to determine formation lithology or the macroscopic capture cross-section, it is often the capture gamma rays that carry most of the information. However, elements of the various components that are in, or which form, the borehole (e.g., drilling fluid, casing, cement, production tubing, and the logging tool itself) also capture neutrons, and thus it is difficult in the related art to distinguish capture gamma rays originating within the borehole from those originating within the formation.